The invention relates to a method for encapsulating a chip on a carrier whereby.
the chip is positioned on the carrier
a dam is formed on the carrier around the chip
encapsulating material is poured into the space within the dam
the obtained encapsulated structure is cured.
A prior art method of this type is described in JP-04302457. According to this method the dam is a premanufactured separate framework which is positioned on the carrier such that it surrounds the chip and is adhered to the carrier by means of an adhesive. Thereafter the encapsulating material is poured into the space within the dam. Premanufacturing, storing and handling of the frameworks is considered expensive and laborious. The premanufacturing process and the encapsulating process are carried out separately and in different processing stations. All this is considered as a serious disadvantage of this method.
Also JP-61101054 obviously describes the use of a premanufactured dam comprising two sections, one sections made of a thermosetting resin melting at high temperature and the second section made of a thermosetting resin melting at low temperature. The second section functions as an adhesive element for positioning and fixing both sections to the substrate after which the encapsulating material is poured into the space within the dam. This method has the same disadvantages as remarked in the preceding paragraph.
In JP 04303989 a method is described for reducing solder areas by forming a dam around the solder areas. On a substrate a lower conductor layer is formed. Where necessary insulating material is applied and thereafter an upper conductor layer is formed. Together with the. upper conductors locally dams are formed on the lower conductors to mark out the restricted soldering areas. The upper. conductors and the dams are constituted as printed layers by applying a suitable printing paste and curing the paste by backing. In a separate soldering step a component is soldered to the lower conductors whereby the amount of solder as well as the soldering areas are delimited.
Another method is known from U.S. Pat. No. 4,961,886. In this known method use is made of a material which will be cured under influence of radiation, for instance ultraviolet radiation, electron beam radiation, or radiation in the visible part of the spectrum. By means of screening means and focusing means care is taken that initially only a part of the carrier is irradiated, i.e. the part where the dam has to be built. Thereafter the encapsulating material is poured onto the carrier at a location within the dam. As soon as the material reaches the position of the dam it will be cured at least partly under the influence of the present radiation and in that way will form the dam. Within the dam initially the material is not cured. As soon as the dam is formed also the area within the dam is irradiated and the whole material is cured.
This prior art method has a number of disadvantages. In the first place this method can only be. applied in combination with an encapsulating material which cures under the influence of radiation. Because of that the choice of the encapsulating material is relatively restricted. A further disadvantage relates to the fact that rather complicated focusing and screening measures are necessary to assure that the radiation from the radiation source will only irradiate that part of the carrier where the dam has to be formed. A further disadvantage relates to the fact that the pouring channel, through which channel the encapsulating material is poured onto the carrier, forms a hindrance for correctly radiating those parts of the carrier where the dam has to be formed. In U.S. Pat. No. 4,961,886 various solutions for this problem are indicated, all of them, however, necessitating additional measures and making therefore the necessary apparatus more complicated.
A method showing a lot of resemblance with the one discussed in the preceding two paragraphs is described in EP-0319175. Curable resin material is poured onto a substrate. A predefined section of the substrate, more specifically the section were the chip is positioned, is irradiated by a suitable source and as soon as the poured resin material reaches the irradiated section the material is cured and a dam is forced around the chip. Thereby the central part of the chip inside the dam is not encapsulated to enable for instance chemical detection by means of a chemically active part of the chip.
The object of the invention is now to indicate how the method defined in the first paragraph for encapsulating a chip on a carrier using the formation of a dam can be realized cheaper and simpler.
In agreement with said object the invention now provides a method with the characteristic
that in the first separate step on the carrier only the dam is formed from a thermohardening material at a relatively high temperature and is cured at least partly and
that in a succeeding second step the space within the dam is filled with the same thermohardening material at a relatively low temperature.
By using a relatively high temperature in the first step the thermohardening material will rather quickly be cured, at least partly, and will form a dam of sufficient strength. In the second step the space within the dam is filled at a relatively low temperature, so that the material within the dam at least initially will be fluid and will therefore have the opportunity to flow out uniformly and fill the whole space within the dam equally.
Preferably the method is carried out in such a manner that in a further third step the obtained encapsulated structure, is cured at a third temperature. Depending on the applied encapsulating material said third temperature is preferably selected such that both the wall as well as the material within the wall will be cured uniformly and preferably tensionless.
Although there are various methods to apply thermohardening material at a desired position on a carrier it is preferred that for the formation of the dam use is made of a first volumetric dispenser dispensing the thermohardening material. Suitable volumetric dispenser units are known as such. Within the possible range of apparatuses it is preferred that the first volumetric dispenser comprises a dispensing mouth which can be moved in two mutually perpendicular directions at some distance above the carrier such that a strip of material with the width and height of the dam can be dispensed.
With relation to the applied temperatures it is preferred that the temperature during the first step is selected depending on the applied thermohardening material such that the material, which is applied in the shape of a dam, after flowing out of the dispensing mouth shows no or only a small amount of shape alteration. Preferably both the carrier as well as the direct surroundings thereof have the desired temperature.
During the second step preferably both the carrier as well as the direct surroundings thereof have the desired relatively lower temperature.
In both steps it is furthermore preferred to take measures, if necessary, to prevent curing of the encapsulating material within the dispensing mouth of the dispensing unit. Such (cooling) measures are, however, known as such.